(1) Technical Field
This invention relates to electronic circuits, and more particularly to radio frequency electronic circuits and related methods.
(2) Background
A simple radio system generally operates in one radio frequency (RF) band for transmitting RF signals and a separate RF band for receiving RF signals. An RF band typically spans a range of frequencies (e.g., 10 to 100 MHz per band), and actual signal transmission and reception may be in sub-bands of such bands, spaced apart to avoid interference. Alternatively, two widely spaced RF bands may be used for signal transmission and reception, respectively.
More advanced radio systems, such as some cellular telephone systems, may be operable over multiple RF bands for signal transmission and reception, but at any one time still use only one transmit sub-band and one receive sub-band within a single RF band, or only two widely spaced transmit and receive RF bands. Such multi-band operation allows a single radio system to be interoperable with different international frequency allocations and signal coding systems (e.g., CDMA, GSM). For some applications, international standards bodies have labeled common frequency bands with band labels, Bn, such as B1, B3, B7, etc. One listing of such bands may be found at https://en.wikipedia.org/wiki/UMTS_frequency_bands.
In recent years, a technique called “Carrier Aggregation” (CA) has been developed to increase bandwidth for RF radio systems, and in particularly cellular telephone systems. In one version of CA known as “inter-band” mode, cellular reception or transmission may occur over multiple RF bands simultaneously (e.g., RF bands B1, B3, and B7). This mode requires passing the receive or transmit RF signal through multiple band filters simultaneously, depending on the required band combination.
FIG. 1A is a block diagram of a prior art RF signal switching and filter circuit 100 that may be used in a CA radio system. In the illustrated example, an antenna 101 is coupled to a multi-path switch 102 that is further coupled to several RF band filters 104. The multi-path switch 102 can selectively couple the antenna 101 to the RF band filters 104 one at a time or in selected combinations. The multi-path switch 102 would typically be implemented using field-effect transistors (FETs), in known fashion. Some or all of the RF filters 104 would be coupled to other RF circuitry, such as a receiver, a transmitter, or a transceiver (not shown). In the illustrated example, band filters 104 for three frequency bands B1, B3, B7 are shown. In operation, the component RF band filters 104 (e.g., for RF bands B1, B3, B7) may be switched into circuit by the multi-path switch 102 individually in a non-CA mode, or in combinations in a CA mode.
For optimum performance, each of the band filters 104 and their desired combinations (e.g., B3 alone, B1+B3 concurrently, and B1+B3+B7 concurrently) must be impedance matched to the switch 102 and antenna 101, typically at a characteristic impedance of 50 ohms for modern radio circuits. FIG. 1B is a Smith chart 110 showing the range of unmatched impedance values of several example combinations of three modeled filters for the configuration shown in FIG. 1A. In the illustrated example, looking at the B3 frequencies only swept over a frequency range of 1.810 GHz to 1.880 GHz in 10 MHz steps, the plot points (for B3 alone, plus the effects of adding B1 or B1+B7 to B3) show that different amounts of impedance matching would be required to match a characteristic impedance of 50 ohms not only for each combination, but also for each frequency step. Accordingly, because of the impedance mismatch, the RF signal switching and filter circuit 100 is not a practical solution for a CA radio system.
If the number of combinations of bands Bn is small and the bands are far enough apart, the band filters 104 may be combined into a single feed point (i.e., no switch 102 is necessary) using passive combining techniques, such as “diplexing” or “triplexing” circuits, which use carefully tuned fixed matching networks to combine multiple filters together and approximately match impedances. For example, FIG. 2 is a block diagram of a prior art RF triplexer filter circuit 200. A bank of filters 104 is connected to an antenna 101 through various fixed combinations of inductors Ln and capacitors Cn that are designed to match the impedance of a respective filter 104 to the impedance of the antenna 101 for a specific band of frequencies (e.g., B1, B3, B7). All of the fixed matching circuit elements must be designed to complement each other. However, such an architecture prevents changing the band combinations selectively and is not practical for more than two or three bands.
To resolve this issue with a small number of frequency bands, it is possible to passively combine separate groups of band filters (e.g., Group1=B1+B3+B7, Group2=B34+B40, and Group3=B38) and then selectively activate one corresponding passively combined impedance matching circuit at a time using a single-pole, multi-throw (SPnT) switch (e.g., SP3T). However, this approach is still not flexible and must be custom designed for every combination of frequency bands. Furthermore, it is essentially not practical to use passive combining for a large number of frequency bands Bn because of the large number of possible combinations of such bands and of overlapping or adjacent frequency ranges.
Accordingly, there is a need for an ability to flexibly combine multiple frequency bands in an RF signal switching and filter circuit that may be used in a CA radio system, without degrading system performance. The present invention addresses this need.